The present invention relates to a conditioning means for printed circuits. More particularly, it relates to a conditioning means for a basis material for producing printed circuits.
The development in the field of the printed circuit technology during the last several years is characterized by a raging boom which first of all is connected with the tendency to miniaturization. This led to further compression of the wiring circuits which are provided on double-side printed circuit boards with important impression provided in complex multi-layer circuits. Switching conductor trains and switching planes are pressed narrowly against one another and are separated from one another by isolating distances which frequently amount only to 80-150 .mu.m.
In contrast to these developments, the basic material remains in principle unchanged. It is composed of glass fiber reinforced epoxy resin which is provided with filler materials for its reinforcement and on the ground of flame confinement. In the conventional working methods there are growing difficulties in maintaining the remaining thin isolating layers free from damaging influence of for example aggressive media for chemical treatment or metallization. If the distances in the older circuits are measured so that the edge action can be tolerated, in highly miniaturized and space economical modern circuits there is no place available for this. Solutions which not only influence the treatment surface but also act in uncalculable manner in the interior of the material, reduce the insulating action between the bore holes respectively between the bore hole and the switching plane and threaten the whole electric operation of the expensive printed circuits.
In the conditioning as preparation for metallization, the activator plays a central role. Because of it, palladium particles are adsorbed on the isolator surfaces of bore holes to be metallized so as to form the seed points for the following chemical metallization. Conventionally, this process takes place in strongly hydrochloric palladium chloride and tin-II-chloride mixed solution, in which under oxidation of the 2-valent to 4-valent tin the reduction to elementary palladium is performed.
The process has become known which involves hydrochloric medium, such as halogen containing solutions, providing highly damaging action, which is known under the name "wicking" (wick formation) or "replating" (return copper plating). It is based on the fact that by hydrochloric acid or other halogenates each bridge of the glass layer in reverse to its formation is always destroyed so as to produce the bond between glass and resin in a basis material. A capillary is formed, in which the halogen-containing solution penetrates along the glass fiber so that it is moved out of the resin in a wick-like manner. Simultaneously, the activation deeply propagates into the interior of the material, so that "rearwardly" or in other words behind the treatment surface a copper plating also occurs. The higher is the glass web contents of the basic material type, the greater is the damaging action.
A further disadvantage stems from the tin compounds. During a long activation time which is required for the rear layer on the glass for coating produced with a delay, the copper of the end front "overactivates" a cut inner layer. By hydrolizing tin compounds which because of their glutinous consistency adhere to the outer surface, this zone is coated with an especially high amount of palladium seeds, so that an inhibiting layer is formed which prevents a firm bond of the inner layer and the metal sleeve of the bore hole. This is a cause for a separation (post separation) after the thermal loading of the soldering.
When for improvement of the insulating properties of the epoxy resin intermediate layers of polyamide are used as is increasingly typical for rigid/flexible circuits, then in acid halogen-containing medium no adherence can be expected since polyamide in lower pH region is completely indifferent.
These disadvantages must be completely eliminated. When the ether bridges of the glass layers are not attacked, the bond glass/resin is produced in full and thereby the electrical isolation resistance inside the printed circuit board is achieved. The same is true for the delamination, or in other words when copper oxide must not be dissolved. Finally, polyamide which is used increasingly as intermediate layer material must be introduced in the same conditioning mechanism.